This invention relates generally to scintillation detectors which are used, for example, to measure radiation at successive depths of bore holes in the earth and, more particularly, to a scintillation detector which has improved performance and durability.
Scintillation detectors include a scintillation crystal which emits light when subjected to an ionizing particle. The crystal is surrounded by a reflector which receives and reflects light from the crystal. The crystal and reflector are enclosed in a housing having a window which is translucent to the light produced by the crystal. The light from the crystal is then passed through the window into a photomultiplier tube and converted to an electrical signal. This process can be used to identify the type and quantity of isotopes that are present.
The crystals which are used in such detectors are frequently metal halide crystals, such as thallium-activated metal halide crystals. It has been known that these metal halide crystals are frequently hygroscopic and absorb water on their surfaces from the air. This causes a film to develop on the crystal surface which degrades the reflectivity of the crystal walls and causes a severe reduction of light output. This reduction of light output adversely affects the performance of the crystal. To prevent this, the detectors are assembled within a dry box to reduce exposure to moisture and the scintillation crystals are typically hermetically-sealed in metal containers or housings.
U.S. Pat. No. 4,004,151, assigned to the assignee of the present invention and incorporated herein in its entirety, discloses a scintillation detector of the type to which this invention relates. This reference describes a detector including a tubular case which encapsulates a scintillation crystal. The tubular case is closed by a plug or cap at one end and the crystal is spring-loaded toward a window at the other end with which the crystal is optically coupled. This spring loading allows for thermal expansion of the crystal relative to this case resulting from the higher coefficient of thermal expansion of the crystal than the case material. In addition, the patent describes in considerable detail, the types of scintillation crystals employed in such devices, their operation, and requirements.
U.S. Pat. No. 4,158,773, also assigned to the assignee of the present invention and incorporated herein in its entirety, discloses a scintillation detector having a special light transfer and reflector means comprising a soft, elastic, silicone, rubber sleeve to improve shock absorption for the protection of the crystal while enhancing light reflection of the crystal.
U.S. Pat. No. 4,383,175, also assigned to the assignee of the present invention and incorporated herein in its entirety, again discloses a similar scintillation detector which has an improved, hermetically-sealed housing and window assembly to reduce the exposure of the crystals to moisture.
One of the major uses of scintillation detectors is in the oil well industry. The detectors are lowered into the bore hole along with other instruments to collect geological data on rock strata to determine whether the well is going to be an effective producer. The process is commonly known as logging the well. During its excursion down the bore hole, the instrument package may be exposed to shock loads as high as 100 to 150 g, and temperatures as high as 200.degree. C. For these reasons, the crystals are mounted in the containers with special shock mounting techniques to prevent damage to the crystal.
The present invention has isolated and met the major problems encountered with this use of the detectors. These problems are that the high-shock and vibration loads will cause the reflective media inside the detector to shift and that subsequent prolonged exposure of the detector to temperatures in the 150.degree. C. to 250.degree. C. range causes a brown film to develop on the crystal surface. This degrades the performance of the detector. It is believed that one reason why these problems have not been understood is that the scintillation detectors are so inaccessible during use and it was easy to place the blame generally for decreased performance on mechanical and thermal trauma encountered during placement and use.
The oil well logging crystal is typically, a long, narrow configuration coupled to the exit window at one end of the housing. The window is coupled to the photo tube. The performance of this configuration is dependent to a large extent on uniform light production along its entire length. Shifting of the powder reflectors, due to excess vibration, upsets the light balance and causes degraded performance. To solve this problem, Teflon tape, a high temperature plastic with good reflective qualities has been tried as a reflector. This material provides a very effective stationary reflector which allows for crystal expansion under heat and doesn't shift under shock and vibration. However, a disadvantage to the tape is that the long-term temperature degradation is magnified and light output can drop as much as 60 percent.
This invention inhibits the degradation that occurs with both powder reflectors and with Teflon tape.